CORTICAL MECHANISMS UNDERLYING TACTILE DISCRIMINATION IN THE MONKEY I. ROLE
OF PRIMARY SOMATOSENSORY CORTEX IN PASSIVE TEXTURE DISCRIMINATION.
F. Tremblay, S. A. Ageranioti-Belanger and C.E. Chapman.
Centre de recherche en sciences neurologiques, Departement de physiologie
and ecole de r[acute]eadaptation, Faculte de m[acute]edecine, Universite de
Montreal, Montreal, Quebec & ecole des sciences de la readaptation,
Universit[acute]e d'Ottawa, CANADA.
APStracts 3:0160N, 1996.
SUMMARY AND CONCLUSIONS
1. The discharge patterns of 359 single neurones in the hand representation of
primary somatosensory cortex (SI) of two monkeys ( macaca mulatta ) were
recorded during the performance of a passive texture discrimination task with
the contralateral hand (104 in area 3b, 149 in area 1, and 106 in area 2).
Three nyloprint surfaces were mounted on a drum that was rotated under the
digit tips. One surface was entirely smooth , while the other two were smooth
over the first half and rough over the second half, smooth/rough (raised dots,
1 mm high, 1 mm in diameter, in a rectangular array; spatial period of 3 mm
across the rows and columns for most recordings; 9 mm between rows for
selected recordings). The monkeys were trained to distinguish between the
smooth and smooth/rough surfaces. After the surface presentation, the monkey
indicated the texture of the second half of the surface by, respectively,
pushing or pulling on a lever with the other arm. For most recordings an
average tangential speed of 49 mm/s was tested. For selected recordings, motor
speed was incremented (63, 75 or 89 mm/s). 2. Two hundred and eighty three
neurones had a cutaneous receptive field (RF) on the hand (96 in area 3b, 120
in area 1, and 67 in area 2). Thirty-five neurones had a deep RF (4 in area
3b, 15 in area 1, and 16 in area 2). Seven neurones had mixed cutaneous and
deep RFs (4 in area 1, 3 in area 2). Thirty-four neurones had no identifiable
RF (4 in area 3b, 10 in area 1, and 20 in area 2). 3. The discharge of 185/359
neurones was significantly modulated during the presentation of one or both
surfaces, as compared to the discharge at rest. Cells with a cutaneous RF that
included part or all of the distal phalangeal pads of the digits used in the
task (usually digits III and IV) were more likely to be modulated during
surface presentation (132/179, 74%) than those with a cutaneous RF not in
contact with the surfaces (24/104, 23%). The remaining neurones (mixed, deep
or no RF) were also infrequently modulated (29/76, 38%). 4. One hundred and
eighteen cells (out of the 185 modulated units) were classified as texture-
related because there was a significant difference in the discharge rate
evoked by the smooth/rough and smooth surfaces. Cells with a cutaneous RF that
included the digital pads in contact with the surfaces were frequently
texture-related (100/132, 76%). Texture-sensitivity was less frequently
observed in the remaining modulated neurones (18/53, 34%: cutaneous RF not in
contact with the surfaces, deep RF, mixed cutaneous and deep RF, no
identifiable RF) 5. Texture-related neurones were found in areas 3b, 1 and 2.
Two patterns of texture-related responses were observed in the 100 cutaneous
units with a RF in contact with the surfaces. Thirty one units were classified
as showing a phasic response at the time the digits encountered the leading
edge of the rough half of the surface. Fifty eight cells were classified as
phasic-tonic (or sometimes tonic at the slowest motor speeds) since the
response lasted for the duration of the presentation of the rough portion of
the surface. The remaining 11 neurones could not be readily classified into
one or the other category and, indeed, generally showed clear texture-related
responses only at higher motor speeds (>49 mm/s, 9/11). 6. Speed sensitivity
was systematically evaluated in 41/100 texture-related units with a cutaneous
RF in contact with the surfaces. The discharge of 66% of the units (27/41)
varied significantly with the speed of surface presentation, with discharge
increasing at higher speeds. Speed-sensitivity was found in all three
cytoarchitectonic areas (6/6 in area 3b, 11/22 in area 1, and 10/13 in area
2). 7. Contact force was also systematically monitored in these experiments
(69/100 texture-related cells with a cutaneous RF in contact with the
surfaces). Linear regression analyses indicated that 22% (15/69) of the
texture-related units were sensitive to contact force (13 positive, 2
negative). For some of these units, however, discharge also covaried with
speed (8/10). Since contact force was not always clearly independent of speed,
the relative importance of contact force in determining the discharge pattern
of neurones involved in the appreciation of surface texture remains unclear.
8. The present results suggest that texture is a distributed function across
areas 3b, 1 and 2. Phasic-tonic units likely provide information about the
characteristics of the scanned textures, while the phasic responses may
represent a form of feature-selectivity in SI cortex. Neurones that signal
differences in the texture and yet are invariant for speed may provide the
neuronal basis for the perceptual constancy of texture across a range of
Received 19 December 1996; accepted in final form 24 June 1996.
APS Manuscript Number J852-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 21 August 1996